Polarizing plate, and liquid crystal display

ABSTRACT

A polarizing plate is provided which includes a polymer film, a polarizer, a polymer substrate, and an optically anisotropic layer containing a liquid crystal compound, laminated in this order, wherein the polarizer has a thickness of 10 to 25 μm. A liquid crystal display using the polarizing plate shows high display quality without causing light leakage.

TECHNICAL FIELD

The present invention relates to a polarizing plate, and a liquidcrystal display using the polarizing plate.

BACKGROUND ART

Liquid crystal display is constituted of a polarizing plate and a liquidcrystal cell.

The most widely used TFT liquid crystal display of TN-mode has anoptical compensation film inserted between the polarizing plate and theliquid crystal cell to realize a high display quality of the liquidcrystal display as described in JP-A-8-50206. However, such a displayhas the disadvantage that the thickness of the liquid crystal displayitself increases.

JP-A-1-68940 describes that use of an elliptic polarizing plate having aretardation film on one side of a polarizer and a protective film on theother side thereof improves front-face contrast of the liquid displaywithout increasing the thickness thereof. However, it was apparent thatthe retardation film (optical compensation film) of the inventiondisclosed in JP-A-1-68940 is liable to cause retardation by thermalstrain or other causes, and thus has the problem on durability. Thisretardation causes framelike light leakage (increase of transmittance)to deteriorate the display quality of the liquid crystal display.

To suppress occurrence of the retardation by strain, JP-A-7-191217 andEP-0911656A2 directly use an optical compensation film formed byapplying an optical anisotropic layer comprising a discotic compound ona transparent support as a protective film for the polarizing plate,thereby solving the above-described problems on the durability withoutincreasing thickness of the liquid crystal display.

However, it was found that when the polarizing plate using the opticalcompensation film as the protective film was set on a large panel of17-inch or larger, it is impossible to completely prevent the lightleakage caused by the thermal strain. The optical compensation filmshould have not only the function of compensating optically the liquidcell but also sufficient durability against change of serviceenvironments.

DISCLOSURE OF THE INVENTION

One object of the present invention is to optically compensate a liquidcrystal cell by using an optical compensation sheet.

Another object of the present invention is to provide a liquid crystaldisplay giving high quality display without light leakage by using anoptical compensation sheet placed on one side of a polarizer.

Still another object of the present invention is to remarkably improvethe production yield of the polarizing plate.

An optical anisotropic layer formed from a liquid crystal compound isused for optically compensating a liquid crystal cell. Generally, theoptical anisotropic layer is provided on a polymer substrate (opticalcompensation film), and a polarizer is interposed between the opticalcompensation film and a triacetylcellulose film as a protective film.

When the optical compensation sheet is used in a liquid crystal display,the optical compensation sheet used in a liquid crystal display isusually fixed to a liquid crystal cell or the like using apressure-sensitive adhesive or the like. Therefore, the strain producedby expansion or contraction of a polymer film of the opticalcompensation sheet is retained in the entire of the optical compensationsheet, changing the optical properties of the polymer film.

Changes of the optical properties have conventionally been considered tobe mainly resulted from the following causes. One cause is variation ofhumidity and temperature conditions in the service environment of theliquid crystal display, resulting in expansion or contraction of thepolymer film to give change of the optical properties of the opticalcompensation sheet. Another cause is nonuniform temperature distributionin the plane of the optical compensation sheet resulting from backlightillumination or the like of the liquid crystal display, resulting inthermal strain to give change of the optical properties of the opticalcompensation sheet.

In particular, it is known that the polymers having hydroxyl groups suchas cellulose esters are greatly influenced by environmental conditions.

Therefore, it has hitherto been believed that the light leakage can beprevented by suppressing the variation of the optical properties of theoptical compensation sheet under the environmental conditions, anduniformizing the temperature distribution in the optical compensationsheet.

As a result of extensive investigations by the present inventors, animportant cause has been found on the variation of the opticalproperties of the optical compensation sheet under environmentalconditions.

Generally, a polarizing plate comprises a pair of protective films and apolarizer comprising PVA as a main component. It has been found that PVAused in the polarizer causes the largest dimensional change due tovariation of humidity and temperature in the service environment of theliquid crystal display. In particular, in a practical liquid crystaldisplay, since the polarizing plate is bonded to a liquid crystal cellthrough a pressure-sensitive adhesive, the dimensional change caused bythe environment is transmitted as deformation stress to the protectivefilm (i.e., optical compensation sheet). This stress will causevariation of the optical properties of the protective film.

Accordingly, it has been found that the strain caused by environment canbe reduced by decreasing the stress ((strain)×(sectionalarea)×(elasticity modulus)) due to the dimensional change of thepolarizer, specifically by decreasing the thickness, and that the lightleakage can remarkably be reduced by decreasing the elastic modulus.

The objects of the present invention have been achieved by the polarizerand the liquid crystal display described below.

-   (1) A polarizing plate comprising a polymer film, a polarizer, a    polymer substrate, and an optically anisotropic layer formed of a    liquid crystal compound, laminated in this order, wherein the    polarizer has a thickness of 10 to 25 μm.-   (2) The polarizing plate described in the above item (1), wherein    the polymer substrate has a thickness of 30 to 70 μm.-   (3) The polarizing plate described in the above item (1) or (2),    wherein the polymer film comprises cellulose acetate.-   (4) The polarizing plate described in the above items (1) to (3),    wherein the polymer substrate comprises cellulose acetate.-   (5) The polarizing plate described in the above items (1) to (4),    wherein the liquid crystal compound used in the optical anisotropic    layer is a discotic liquid crystal compound, the plane of the    discotic structural units is inclined relative to the surface of the    polymer substrate, and the angle between the plane of the discotic    structural units and the surface of the polymer substrate changes in    the direction of the depth of the optically anisotropic layer.-   (6) A liquid crystal display comprising a liquid crystal cell, and    two polarizing plates placed on both sides of the liquid crystal    cell, wherein at least one polarizing plate is the polarizing plate    described in items (1) to (5).-   (7) The liquid crystal display described in the above item (6),    wherein the liquid crystal cell is of an OCB mode, a VA mode, or a    TN mode.

BEST MODE FOR CARRYING OUT THE INVENTION

The polarizing plate of the present invention has one characteristic inthat a polarizer used therein has a thickness of 10 to 25 μm.

Polarizer:

The polarizer of the present invention comprises a binder, and iodine ora dichroic dyestuff.

The iodine and the dichroic dyestuff in the present invention areoriented in the binder, whereby the polarizing plate of the presentinvention exhibits the polarizing performance. The iodine and thedichroic dyestuff are oriented along the binder molecules.

The polarizer is prepared by immersing a stretched polymer in a bathcontaining iodine or a dichroic dyestuff to permeate the iodine or thedichroic in the binder.

In a commercially available polarizer, the iodine or the dichroicdyestuff is distributed in the regions of about 4 μm inside from thepolymer surfaces (about 8 μm in total on the both sides). Therefore, athickness of at least 10 μm is necessary to obtain a sufficientpolarizing performance. The degree of permeation can be controlled bythe concentration of the dissolved iodine or dichroic dyestuff, the bathtemperature, and the immersion time.

The lower limit of the thickness of the binder in the present inventionis preferably at least 10 μm from the above-described reasons. The upperlimit of the thickness is preferably as small as possible in view of thelight leakage of the liquid crystal display, and the thickness should benot more than that of the commercially available polarizing plate (about30 μm), preferably 25 μm or less. The light leakage can be prevented ina 17-inch liquid crystal display by decreasing the thickness to 20 μm orless.

The binder used is not particularly limited, and may be aself-crosslinkable polymer, or may be a binder crosslinkable by acrosslinking agent. The binder layer can be formed by reaction betweenbinder molecules having a functional group or having an introducedfunctional group, under action of light, heat, pH change, or the like;or by introducing bonding groups between binders using a crosslinkingagent that is a compound having high reactivity, and crosslinking thebinders.

The crosslinking can be conducted by applying a coating liquidcontaining the binder or a mixture of the binder and a crosslinkingagent to a transparent support, and then heating or the like. Thecrosslinking treatment may be conducted in any stage until obtaining thefinal polarizer so long as the durability can be secured at the stage offinal commercial article.

The binder used in the present invention can be either aself-crosslinkable polymer or a polymer crosslinkable by a crosslinkingagent. Of course, there are binders self-crosslinkable and alsocrosslinkable by a crosslinking agent. Examples of the binders includepolymers such as polymethyl methacrylate, acrylic acid/methacrylic acidcopolymers, styrene/maleimide copolymers, polyvinyl alcohols, modifiedpolyvinyl alcohols, poly(N-methylol-acrylamide), styrene/vinyltoluenecopolymers, chlorosulfonated polyethylene, nitrocellulose, polyvinylchloride, chlorinated polyolefins, polyesters, polyimides, vinylacetate/vinyl chloride copolymers, ethylene/vinyl acetate copolymers,carboxymethylcellulose, polyethylene, polypropylene, and polycarbonate;and compounds such as silane coupling agents. Examples of the polymerspreferably are water-soluble polymers such aspoly(N-methylolacrylamide), carboxymethylcellulose, gelatin, polyvinylalcohol, and modified polyvinyl alcohol. Of those, gelatin, polyvinylalcohol, and modified polyvinyl alcohol are more preferable, andpolyvinyl alcohol and modified polyvinyl alcohol are especiallypreferable.

Of the above binders, polyvinyl alcohols and modified polyvinyl alcoholsare preferable. The polyvinyl alcohol has, for example, a degree ofsaponification of 70 to 100%, preferably 80 to 100%, and more preferably95% or more, and has a degree of polymerization in range of preferably100 to 5,000. Examples of the modified polyvinyl alcohol include theones modified by copolymerization (the modifying groups including COONa,Si(OH)₃, N(CH₃)₃C₁, C₉H₁₉COO, SO₃Na, and C₁₂H₂₅), the ones modified bychain transfer (the modifying groups including COONa, SH, and C₁₂H₂₅),and the ones modified by block polymerization (the modifying groupsincluding COOH, CONH₂, COOR (alkyl), and C₆H₅). The modified polyvinylalcohol preferably has a degree of polymerization of 100–3,000. Ofthose, unmodified and modified polyvinyl alcohols having a degree ofsaponification of 80 to 100% are preferable, and unmodified andalkylthio-modified polyvinyl alcohols having a degree of saponificationof 85 to 95% are more preferable.

The polyvinyl alcohol or the modified polyvinyl alcohols used in thebinder layer may be used alone or in combination of two or more thereof.

Compounds disclosed in JP-A-8-338913, JP-A-9-152509, and JP-A-9-316127are particularly preferably used as the modified polyvinyl alcohol.

The crosslinking agent for the binder is not particularly limited. Alarger addition amount of the crosslinking agent tends to give moreimprovement in resistance to high humidity and high temperature.However, addition of the crosslinking agent in an amount of 50 mass % ormore, based on the binder, lowers the orientation property of the iodineor the dichroic dyestuff. Therefore, the addition amount is preferably0.1 to 20 mass %, more preferably 0.5 to 15 mass %. The alignment filmof the present invention contains unreacted crosslinking agent to someextent after completion of the crosslinking reaction. The amount of theremaining crosslinking agent is preferably 1.0 mass % or less, morepreferably 0.5 mass % or less. If the crosslinking agent is contained inan amount of more than 1.0 mass % in the binder layer, sufficientdurability cannot be achieved. Specifically, when such an alignment filmis used in a liquid crystal display, deterioration of the polarizationperformance may occur during long-term service, or long-term storageunder high temperature and high humidity atmosphere.

The specific examples of the crosslinking agent are disclosed in U.S.Re. Pat. No. 23,297. Of those, boric acids (boron, and borax) arepreferably used.

Iodine and Dichroic Dyestuff:

The dichroic molecule includes dyestuff compounds such as azo dyestuffs,stilbene dyestuffs, pyrazolone dyestuffs, triphenylmethane dyestuffs,quinoline dyestuffs, oxazine dyestuffs, thiazine dyestuffs, andanthraquinone dyestuffs. The dyestuff is preferably water-soluble, butis not limited thereto. These dichroic molecules have preferably ahydrophilic substituent introduced thereto, such as a sulfonic acidgroup, an amino group, and a hydroxyl group. Examples of the dichroicmolecule include C.I. Direct Yellow 12, C.I. Direct Orange 39, C.I.Direct Orange 72, C.I. Direct Red 39, C.I. Direct Red 79, C.I. DirectRed 81, C.I. Direct 83, C.I. Direct 89, C.I. Direct Violet 48, C.I.Direct Blue 67, C.I. Direct Blue 90, C.I. Direct Green 59, and C.I. AcidRed 37; and dyestuffs disclosed in JP-A-1-161202, JP-A-1-172906,JP-A-1-172907, JP-A-1-183602, JP-A-1-248105, JP-A-1-265205, andJP-A-7-261024. Such dichroic molecules are used in a form of a freeacid, or a salt such as an alkali metal salt, an ammonium salt, and anamine salt. Polarizers having various color tones can be prepared byblending two or more of the dichroic molelcules. The compounds(dyestuffs) which give a black color when the polarization axises ofpolarizing elements or polarizing plates are placed perpendicularly, andblends of two or more kinds of dichroic molecules which give black colorare preferred because of excellent single plate high transmittance anddegree of polarization.

From the standpoint of increasing the contrast ratio of the liquidcrystal display, the transmittance of the polarizing plate is preferablyhigher, and the degree of polarization thereof is preferably higher. Thetransmittance of the polarizing plate is in a range of preferably 30 to50%, more preferably 35 to 50%, most preferably 40 to 50%, at a lightwavelength of 550 nm. The degree of polarization is in a range ofpreferably 90 to 100%, more preferably 95 to 100%, most preferably 99 to100%, at a light wavelength of 550 nm.

The transmittance of the polarizing plate can be increased by increasingthe transmittance of the polymer film as described later, or byadjusting the refractive index of the adhesive bonding the polarizer andthe polymer film.

The transmittance of the polymer film as described later can beincreased by decreasing the film thickness or lowering the haze of thefilm.

The adhesive for bonding the polarizer and the polymer film together, orthe polarizer and the optically anisotropic layer together, is notparticularly limited. Examples of the adhesive include PVA type resins(including PVA modified by a group of acetoacetyl, sulfo, carboxyl,oxyalkylene, etc.) and aqueous solution of boron compounds. Of those,PVA type resins are preferred. The thickness of the adhesive is in arange of preferably 0.01 to 10 μm, more preferably 0.05 to 5 μm, afterdrying.

The refractive index of the adhesive is preferably close to that ofcellulose acetate film. The difference in the refractive index betweenthe adhesive and the cellulose acetate film is preferably 0.1 orsmaller, more preferably 0.05 or smaller, most preferably 0.01 orsmaller.

The polymer film and the polymer substrate for interposing the polarizerof the present invention therebetween are explained below.

Polymer Film and Polymer Substrate:

The polymer film used preferably has light transmittance of 80% or more.Examples of the polymer for constituting the film include celluloseesters (e.g., cellulose acetate, and cellulose diacetate), norbornenetype polymers, and polymethyl methacrylate. Commercially availablepolymers may also be used (Artone, and Zeonex as the norbornene typepolymer). Cellulose esters are preferable, and lower fatty acid estersof cellulose are more preferable. The term “lower fatty acid” usedherein means a fatty acid having 6 or less carbon atoms. Of those, thecarbon atom number is preferably 2 (cellulose acetate), 3 (cellulosepropionate), or 4 (cellulose butyrate). Of those, cellulose acetate isparticularly preferred. Mixed fatty acid esters such as celluloseacetate propionate, and cellulose acetate butyrate may be used.

Even conventional polymers that are liable to develop birefringence,such as polycarbonate or polysulfone, can be used by suppressing thetendency by modifying the molecule as described in the patentspecification of WO 00/26705.

Cellulose acetate having an acetic acid content of 55.0 to 62.5%,preferably 57.0 to 62.0% is preferably used as the polymer film.

The term “acetic acid content” used herein means an amount of bondedacetic acid to a unit mass of cellulose. The acetic acid content ismeasured and calculated according to ASTM: D-817-91 (Method of TestingCellulose Acetate).

The viscosity-average degree of polymerization (DP) of the celluloseester is preferably 250 or more, more preferably 290 or more. Thecellulose ester used in the present invention has preferably narrowermolecular weight distribution, Mw/Mn (Mw: mass-average molecular weight,Mn: number-average molecular weight), as measured by gel permeationchromatography. Specifically, the value of Mw/Mn is in a range ofpreferably 1.0 to 1.7, more preferably 1.3 to 1.65, most preferably 1.4to 1.6.

In the cellulose ester, hydroxyl groups at 2-, 3-, and 6-positions arenot uniformly esterified respectively at ⅓ of the entire substitution.The degree of substitution tends to decrease at the 6-position. In thepresent invention, the degree of substitution is preferably higher atthe 6-position than at the 2- and 3-positions.

Specifically, the proportion of substitution of the hydroxyl at6-position accounts for preferably 30–40% of the entire substitution;more preferably 31% or more, most preferably 32% or more. The degree ofsubstitution at the 6-position is preferably 0.88 or more.

The hydroxyl at the 6-position may be substituted by an acyl grouphaving 3 or more carbon numbers other than acetyl (e.g., propionyl,butyryl, valeryl, benzoyl, and acryloyl). The degree of substitution ofthe respective positions can be measured by NMR.

The cellulose ester having hydroxyl groups at 6-position substituted ata higher degree of substitution can be synthesized by reference to themethods described in JP-A-11-5851, paragraphs 0043–0044, SynthesisExample 1; paragraphs 0048–0049, Synthesis Example 2; and paragraphs0051–0052, Synthesis Example 3.

The cellulose acetate film can be produced from the prepared celluloseacetate solution (dope) by a solvent cast method. The dope containspreferably a retardation-increasing agent.

The film is formed by casting the dope onto a drum or a band andevaporating the solvent. The dope for the casting is preferably adjustedto have a solid concentration of 18–35%. The surface of the drum or bandis preferably finished in a mirror state. The casting and drying methodsin the solvent casting method are disclosed in U.S. Pat. Nos. 2,336,310,2,367,603, 2,492,078, 2,492,977, 2,492,978, 2,607,704, 2,739,069, and2,739,070; British Patents 640731, and 736892; JP-B 45-4554, and 49-5614and JP-A 60-176834, 60-203430, and 62-115035.

The dope is cast preferably onto a drum or band having a surfacetemperature of preferably 10° C. or lower. After casting, the dope thuscast is dried by air stream preferably for 2 seconds or longer. Theobtained film is stripped off from the drum or band. The film may befurther dried by a hot air stream at successively raised temperatures offrom 100 to 160° C. to evaporate the remaining solvent. The above methodis described in JP-B-5-17844. By this method, the time from the castingto the peeling can be shortened. For conducting this method, the dopeshould gel at the surface temperature of the drum or band duringcasting.

The film may be formed by casting a prepared cellulose acetate solution(dope) in two or more layers. The dope is cast onto a drum or band andthe solvent is evaporated to form a film. The dope for the casting ispreferably adjusted to have a solid matter concentration of 10–40%. Thesurface of the drum or band is preferably finished in a mirror state.

In the case where two or more cellulose acetate solutions are cast, thecellulose acetate-containing solutions may be cast respectively throughplural casting dies placed at intervals along the support movementdirection to form layers. Such a lamination can be conducted accordingthe method disclosed in, for example, JP-A-61-158414, JP-A-1-122419, andJP-A-11-198285. The cellulose acetate solution may be cast through twocasting dies to form a film by a method shown in, for example,JP-B-60-27562; and JP-A-61-947244, 61-947245, 61-104813, 61-158413, and6-134933. Another casting method of the cellulose acetate film may beused in which a flow of a high-viscosity cellulose acetate solution isenveloped with a low-viscosity cellulose acetate solution and thecellulose acetate solutions are simultaneously extruded.

In forming an optically anisotropic layer comprising a liquid crystalcompound on a polymer film, it is preferable to control the retardationof the polymer film (herein, a polymer substrate).

Retardation of Polymer Substrate:

The preferred range of the retardation of the polymer substrate dependson the kind of the liquid crystal cell using the optical compensationfilm and the use manner thereof. It is preferable in the presentinvention that the Re retardation value is controlled to a range of 20to 70 nm, and the Rth retardation value is controlled to a range of 70to 400 nm.

In the case using two optically anisotropic layers in a liquid crystaldisplay, it is preferable that the Rth retardation value of thesubstrate is controlled to a range of 70 to 250 nm.

In the case of using one optically anisotropic layer in a liquid crystaldisplay, it is preferable that the Rth retardation value of thesubstrate is controlled to a range of 150 to 400 nm.

The birefringent index (Δn: nx−ny) of the substrate is preferably in arange of 0.00028 to 0.020. The birefringent index in the thicknessdirection {(nx+ny)/2−nz} of the cellulose acetate film is preferably ina range of 0.001 to 0.04.

Retardation-Increasing Agent:

For adjusting the retardation of the polymer film, an aromatic compoundhaving at least two aromatic rings is preferably used as aretardation-increasing agent.

Hereinafter, cellulose acetate, as one example, is explained in detailwhich is the best embodiment of the polymer film.

The aromatic compound is used in an amount of 0.01–20 parts by mass,preferably 0.05 to 15 parts by mass, and more preferably 0.1 to 10 partsby mass, per 100 parts by mass of cellulose acetate. Two or morearomatic compounds may be used in combination.

The aromatic ring of the aromatic compound includes aromatic hydrocarbonrings as well as aromatic heterocyclic rings.

Six-membered rings (i.e., benzene ring) are particularly preferred asthe aromatic hydrocarbon ring.

The aromatic heterocyclic rings are generally unsaturated rings. Thearomatic heterocyclic ring is preferably a five-memberd ring, asix-membered ring, or a seven-membered ring, more preferably afive-membered ring or a six-membered ring. The aromatic heterocyclicring has generally the maximum number of double bonds. The heteroatom ispreferably nitrogen atom, oxygen atom, or sulfur atom; particularlynitrogen atom. The aromatic heteroyclic ring includes furan rings,thiophene rings, pyrrole rings, oxazole rings, isoxazole rings, thiazolerings, isothiazole rings, imidazole rings, pyrazole rings, furazanrings, triazole rings, pyran rings, pyridine rings, pyridazine rings,pyrimidine rings, pyrazine rings, and 1,3,5-triazine rings.

Preferable aromatic rings are benzene rings, furan rings, thiophenerings, pyrrole rings, oxazole rings, thiazole rings, imidazole rings,triazole rings, pyridine rings, pyrimidine rings, pyrazine rings, and1,3,5-trizine rings: particularly preferred are benzene rings and1,3,5-triazine rings.

The aromatic compound has preferably at least one 1,3,5-triazine ring.

The aromatic compound has preferably 2–20 aromatic rings, morepreferably 2–12 aromatic rings, further preferably 2–8 aromatic rings,most preferably 2–6 aromatic rings.

Two aromatic rings are linkable by (a) formation of a condensed ring,(b) direct bonding by a single bond, or (c) linkage by a linking group(spiro bonding cannot be formed due to aromatic rings). The rings may belinked in any of the above manners (a) to (c).

Such retardation-increasing agents are described in, for example, WO01/88574A1, WO 00/2619A1, JP-A-2000-111914, and JP-A-2000-275434.

The retardation of the cellulose acetate film can also be adjusted bystretching treatment. The stretch ratio is preferably in a range of 3 to100%. The stretching of the cellulose acetate in the present inventionis preferably conducted by tenter stretching. For fine control of thephase retarding axis, the difference in the tenter-clipping rates at theboth sides and the release timing is preferably minimized. Thestretching treatment can be conducted according to WO-01/88574A1, page37, line 8 to page 38, line 8.

Surface Treatment of Cellulose Acetate Film:

The cellulose acetate film is preferably subjected to a surfacetreatment. The treatment specifically includes corona dischargetreatment, glow discharge treatment, flame treatment, acid treatment,alkali treatment, and UV irradiation treatment. An under-coating layeris preferably provided as described in JP-A-7-333433.

In the above treatment, the temperature of the cellulose acetate filmshould be Tg (glass transition temperature) or lower, specifically 150°C., from the standpoint of maintaining the flatness of the film.

The surface treatment of the cellulose acetate is preferable an acidtreatment or an alkali treatment, namely saponification treatment, fromthe standpoint of adhesiveness to the polarizer.

The surface energy is preferably 55 mN/m or more, more preferably in arange of 60 to 75 mN/m. The alkali saponification treatment isspecifically explained below as an example.

The alkali saponification treatment is preferably conducted by the cycleof immersion of the film surface into an alkali solution, neutralizationwith an acid solution, washing with water, and drying.

The alkali solution includes a potassium hydroxide solution, and asodium hydroxide solution. The hydroxyl ion concentration thereof is ina range of preferably 0.1 to 3.0N, more preferably 0.5 to 2.0N. Thetemperature of the alkali solution is in a range of preferably from roomtemperature to 90° C., more preferably 40 to 70° C.

The surface energy of a solid can be measured by a contact angle method,a wetting heat method, or an adsorption method as described in theliterature “Nure no Kiso to Ouyou (Wetting: Elements and Applications)”(Riaraizu K. K., 1989.12.10). The surface energy of the celluloseacetate film of the present invention is preferably measured by thecontact angle method.

Specifically, two kinds of liquids having respectively a known surfaceenergy are dropped onto the cellulose acetate film, and the contactangles are measured. The contact angle is an angle between the surfaceof the film and the tangent to the liquid drop at the crossing point ofthe drop surface and the film surface at the liquid drop side. Thesurface energy of the film is calculated from the measured contactangles.

Optically Anisotropic Layer Comprising Liquid Crystalline Compound:

The surface of the support of the polarizing plate of the presentinvention can be provided with a functional layer such as an opticallyanisotropic layer for compensation of the visual field angle of LCD asdescribed in JP-A-4-229828, JP-A-6-75115, JP-A-8-50206, etc.; aglare-preventing or reflection-preventing layer for visibilityimprovement of display; a PS wave-separating layer for LCD luminanceimprovement against anisotropic scattering and anisotropically opticalinterference (polymer dispersion liquid crystal layer, cholestericliquid crystal layer, etc.); a hard coat layer for scratch resistance ofthe polarizing plate; a gas barrier layer for preventing diffusion ofmoisture or oxygen; an adhesion-promoting layer for strengthening thecontact of the polarizer with an adhesive or a pressure-sensitiveadhesive; slipping-imparting layer; and so forth.

The functional layer may be provided on the side of the polarizer, ormay be provided on the side opposite the polarizer, depending on theobject.

Various functional films as a protective film can directly be bonded toone or both faces of the polarizer of the present invention. Examples ofthe functional film include retardation films such as a λ/4 plate, andλ/2 plate; a light-diffusing film; a plastic cell having anelectroconductive layer on the face opposite the polarizing plate; aluminance-improving film having a function of anisotropic scattering oranisotropically optical interference; a reflective film; and asemi-transparent reflective plate.

The polarizing plate of the present invention exhibits its function moreeffectively by combining with a coat type of optical member (e.g., anoptical compensation film, and a luminance-improving film), therebycontrolling the transmission axis of the polarizing plate and theretardation axis of the respective optical members. As specific examplesof the coat type optical member, optical compensating sheets usingdiscotic liquid crystal molecules are described in, for example,JP-A-6-214116, U.S. Pat. No. 5,583,679, U.S. Pat. No. 5,646,703, andGerman Patent 3911620A1; optical compensating sheet using rod-shapedliquid crystalline molecules are described in, for example,JP-A-7-35924; and a luminance-improving film is described in, forexample, JP-A-11-149015.

A preferable embodiment of the anisotropically optical layer comprisingthe liquid crystal compound of the present invention is described indetail below.

An alignment film is preferably provided between the polymer substrateand the optically anisotropic layer. The alignment film functions toalign the liquid crystal compound of the present invention in a constantdirection. The alignment film is essential to realize the preferredembodiment of the present invention. However, after orientation of theliquid crystal compound and fixation of the aligned state, the alignmentfilm is not essential as the structural element of the present inventionbecause the alignment film plays its role. Therefore, the polarizingplate of the present invention can be prepared by transferring only theoptically anisotropic layer on the alignment film having the fixedaligning state to the polarizer.

Alignment film:

The alignment film has a function to define the orienting direction ofthe liquid crystal compound. The alignment film can be formed in variousmethods such as a rubbing treatment of an organic compound (preferably apolymer); oblique deposition of an inorganic compound; formation of alayer containing microgrooves; and building-up of an organic compound(e.g., ω-tricosanoic acid, dioctadecylmethylammonium chloride, andmethyl stearate) by a Langmuir-Blodgett technique (LB film). Otheralignment films are known in which an orienting function is imparted byapplication of an electric field, application of a magnetic field, orirradiation of light. The alignment film is preferably formed by rubbingtreatment of a polymer.

The alignment film is preferably formed by rubbing treatment of apolymer. Polyvinyl alcohol is preferred as the polymer. Modifiedpolyvinyl alcohols having hydrophobic groups bonded thereto areparticularly preferred.

The alignment film is described in, for example, WO 01/88574A1, page 43line 24 to page 49, line 8.

Optically Anisotropic Layer:

The optically anisotropic layer formed from a liquid crystal compound ispreferably prepared on an alignment film provided on a polymer substratein the present invention.

The liquid crystal compound used for the optically anisotropic layerincludes rod-like liquid crystal compounds and discotic liquid crystalcompounds. The rod-like liquid crystal compounds and discotic liquidcrystal compounds may be a polymeric liquid crystal or a low-molecularliquid crystal. The liquid crystal compound also includes low-molecularliquid crystals in which the liquid crystallinity has been lost bycrosslinking.

The optically anisotropic layer can be formed by applying a coatingliquid containing a liquid crystal compound and optionally apolymerization initiator or another component to an alignment film.

In the case of using a discotic liquid crystal compound, it ispreferable that the plane of the discotic structural units is inclinedrelative to the polymer substrate, and the angle between the plane ofthe discotic structural unit and the surface of the polymer substratevaries in the depth direction of the optically anisotropic layer.

The angle (inclined angle) of the plane of the discotic structural unitsis generally increased or decreased depending on the distance from thebottom face of the optically anisotropic layer in the depth direction ofthe optically anisotropic layer. The inclined angle is preferablyincreased with increase of the distance. The change of the inclinedangle may be continuous increase, continuous decrease, intermittentincrease, intermittent decrease, combination of continuous increase andcontinuous decrease, or intermittent change including increase anddecrease. The intermittent change includes a region in which theinclined angle does not change in the course of the thickness direction.Even in the case where no angle-change region is contained in the layer,the inclined angle is preferably increased or decreased as a whole. Morepreferably, the inclined angle is increased as a whole, and still morepreferably the change is continuous.

The optically anisotropic layer is described in WO 01/88574A1, page 49,line 10 to page 67, line 20 as the reference.

Polarizer Having No Transmission Axis in Length Direction and in WidthDirection:

The polarizer of the present invention is prepared by stretching a rawfilm in a direction inclined to the length direction at an angle of 10to 80° (stretching method), or rubbing the film (rubbing method), andthen dyeing the film with iodine or a dichroic dyestuff. Stretching isconducted such that this inclined angle is made equal to the anglebetween the transmission axis of the two polarizing plates bonded to aliquid crystal cell of LCD and the vertical or lateral direction of theliquid crystal cell.

Usually, this angle is 45°. However, in new models of the transmissiontype LCD, the reflection type LCD, the semi-transmission type LCD, andthe like, the angle is not necessarily 45°. The stretch direction isadjusted suitably in accordance with the design of the LCD.

In the stretching method, the stretch ratio is preferably 2.5 to 30.0,more preferably 3.0 to 10.0. The stretching may be conducted by drystretching in the air or by wet stretching in water. The stretch ratiois about 2.5 to 5.0 in dry stretching, and about 3.0 to 10.0 in wetetching. This oblique stretching may be conducted fractionally in pluraltimes. The fractional stretching enables uniform stretching even in ahigh stretch ratio. Before the oblique stretching, slight stretching maybe conducted in the width direction or the length direction (to anextent to prevent the contraction in the width direction).

In the stretching, the tenter stretching for biaxial stretching in usualfilm molding may be conducted differently in the right side and the leftside. That is, the stretching is conducted at different speeds at theleft side and the right side. For such differential stretching, thethickness of the binder film before the stretching is necessarily madedifferent between the right side and the left side of the film. Incasting film formation, for example, the flow rate of the bindersolution can be differentiated between the right side and the left sideby, for example, providing tapers to the die.

In such a process, the polarizer of the present invention is producedwith the stretching direction oblique by 10 to 80° to the lengthdirection.

The rubbing method can be conducted according to a usual rubbingtreatment method employed widely in a liquid crystal orienting processof LCD. Specifically, the surface of the alignment film is rubbed in onedirection with paper, gauze, felt, rubber, fiber of nylon or polyester,or a like material to impart orienting property. Generally, thetreatment is conducted by rubbing the film several times with a clothhaving fibers of uniform length and diameter transplanted uniformly, ora like material. In the rubbing treatment employed preferably in thepresent invention, a rubbing roll is used which has circularity,cylindricity, and eccentricity of the roll of 30 μm or less. Thewrapping angle of the film to the rubbing roll is preferably in a rangeof 0.1 to 90°. However, stable rubbing treatment can be achieved bywinding the film at an angle of 360° or more as disclosed inJP-A-8-160430. For rubbing of a long film, the film is preferablydelivered at a constant tensile force at a rate of 1 to 100 m/min. Therubbing roll is preferably turnable horizontally relative to the filmdelivery direction for setting a desired rubbing angle, the angle beingsuitably selected from a range of 0 to 60°, with 45° being preferable.For use as the liquid crystal display, the angle is preferably in arange of 40 to 50°.

Liquid Crystal Display:

The polarizing plate using the cellulose acetate film can advantageouslybe used for liquid crystal displays, particularly for transmission typeliquid crystal displays.

The transmission type liquid crystal display comprises a liquid crystalcell and two polarizing plate placed on the both sides thereof. Theliquid crystal cell holds a liquid crystal between two electrode baseplates.

The polarizing plate of the present invention can be used as one or bothof the above polarizing plates. In this case, the (opticallyanisotropic) cellulose acetate film of the polarizing plate is placed soas to face the liquid crystal cell.

The liquid crystal cell is preferably of an OCB mode, a VA mode, an ECBmode, or a TN mode.

The liquid crystal cell of the OCB mode is a liquid crystal cell of abend orientation mode in which the rod-like liquid crystal molecules areoriented to become reverse substantially (symmetrically) in theorientation direction between the top and the bottom of the liquidcrystal cell. A liquid crystal display using the liquid crystal cell ofthe bend orientation mode is disclosed in U.S. Pat. Nos. 4,583,825, and5,410,422. The liquid crystal cell of the bend orientation mode has selfoptical compensation function due to the symmetric orientation of therod-like liquid crystal molecules between the top and the bottom of theliquid cell. Therefore, this liquid crystal mode is called an OCB(optically compensatory bend) crystal liquid mode. The liquid crystaldisplay of the bend orientation mode has an advantage of high responsespeed.

The polarizing plate of the present invention, which is used in a liquidcrystal display of an OCB mode, may have an optically anisotropic layercontaining a discotic compound or a rod-like compound on the celluloseacetate film used as the polarizing plate. The optical anisotropic layeris formed by orienting the discotic compound (or the rod-like liquidcrystal compound) and fixing the oriented state.

Discotic compounds have generally a higher birefringent index. Thediscotic compound can take various oriented states. Therefore, by usingthe discotic compound, a polymer film (optical compensation film) can beproduced which has optical properties that cannot be obtained from aconventional stretched birefringent film. The polymer film using thediscotic compound is described in JP-A-6-214116, U.S. Pat. Nos.5,583,679 and 5,646,703, and German Patent 3911620A1.

In the liquid crystal cell of the VA mode, the rod-like liquid crystalmolecules orients substantially vertically when not applying voltage.

The liquid crystal cell in the VA mode includes (1) liquid crystal cellsin a narrow sense in which rod-like liquid crystal molecules are alignedsubstantially vertically when not applying voltage and are orientedsubstantially horizontally when applying voltage (JP-A-2-176625), (2)liquid crystal cells (MVA mode) in which VA mode is formed in multipledomains for visual field angle expansion (SID97, Digest of tech. Papers(Preprint) 28 (1997) 845), (3) liquid crystal cells (n-ASM mode) inwhich rod-like liquid crystal molecules are oriented substantiallyvertically when not applying voltage and are oriented in twistedmultiple domains when applying voltage (Nippon Ekisho Toronkai (JapanLiquid Crystal Symposium), Preprint 58–59 (1998)), and (4) liquidcrystal cells of SURVIVAL mode (presented at LCD International 98).

The ECB mode of the liquid crystal is the oldest of the liquid crystalmodes, and is described in many documents.

In the liquid crystal cell of the TN mode, the rod-like liquid crystalmolecules are oriented substantially vertically when not applyingvoltage in a state twisted at an angle of 60 to 120°.

TN mode liquid crystal cells are used most widely, and are described inmany documents.

EXAMPLES

The present invention is explained in detail by reference to thefollowing Examples, but it should be understood that the invention isnot construed as being limited thereto.

Example 1

Preparation of Polarizer:

A PVA film having an average degree of polymerization of 1700 and adegree of saponification of 99.5 mol % (thickness, 80 μm; width, 2,500mm) was stretched uniaxially in the vertical direction in warm water at40° C. at a stretch ratio of 8. The film in this state was immersed inan aqueous solution containing 0.2 g/l of iodine and 60 g/l of potassiumiodide at 30° C. for 5 minutes, and then immersed in an aqueous solutioncontaining 100 g/l of boric acid and 30 g/l of potassium iodide. Thefilm in this state had a width of 1,300 mm and a thickness of 17 μm.

The film was then immersed in a water-washing vessel at 20° C. for 10seconds, and further immersed in an aqueous solution containing 0.1 g/lof iodine and 20 g/l of potassium iodide at 30° C. for 15 seconds. Thefilm was dried at room temperature for 24 hours to obtain an iodine typepolarizer (HF-1).

Preparation of Polymer Substrate:

The composition shown below was put into a mixing tank, and thecomponents were dissolved by stirring with heating to prepare acellulose acetate solution.

Composition of Cellulose Acetate Solution

Parts by mass Cellulose acetate 100 (acetic acid content: 60.9%)Triphenyl phosphate (plasticizer) 7.8 Biphenyl diphenyl phosphate(plasticizer) 3.9 Methylene chloride (first solvent) 300 Methanol(second solvent) 54 1-Butanol (third solvent) 11

16 Parts by mass of the retardation-increasing agent shown below, 80parts by mass of methylene chloride, and 20 parts by mass of methanolwere introduced into another mixing tank. The resulting mixture wasstirred with heating to prepare a solution of the retardation-increasingagent.

464 Parts by mass of the cellulose acetate solution and 36 parts by massof the retardation-increasing agent solution were mixed and stirredsufficiently to prepare a dope. The amount of the retardation-increasingagent added was 5.0 parts by mass per 100 parts by mass of celluloseacetate.

Retardation-Increasing Agent

The dope obtained was cast using a band-casting machine. When the filmsurface temperature had reached 40° C. on the band, the film formed wasdried for 1 minute. The film was stripped off from the band, and driedby drying air stream at 140° C. The film was stretched by 28% in thewidth direction by a tenter, and then dried with a drying air stream at135° C. for 20 minutes. Thus, a polymer substrate (PK-1) was preparedwhich contained the residual solvent at a content of 0.3 mass %.

The thus-obtained polymer substrate (PK-1) had a thickness of 92 μm. Theretardation value (Re) at 590 nm of the substrate was 43 nm, and theretardation value (Rth) at 590 nm was 175 nm as measured by anellipsometer (M−150, manufactured by Nippon Bunko K.K.).

The thus-prepared polymer substrate (PK-1) was immersed in a 2.0Npotassium hydroxide solution (25° C.) for 2 minutes, then neutralizedwith sulfuric acid, washed with pure water, and dried. This PK-1 had asurface energy of 63 mN/m, as measured by a contact angle method.

This PK-1 was coated with an alignment film-coating liquid having thecomposition shown below in an application amount of 28 ml/m² with a #16wire bar coater. The coated film was dried by warm air stream at 60° C.for 60 seconds and further at 90° C. for 150 seconds.

Composition of Alignment Film Coating Liquid

Parts by mass Modified polyvinyl alcohol shown below 10 Water 371Methanol 119 Glutaraldehyde (crosslinking agent) 0.5Modified Polyvinyl Alcohol

The formed alignment film was subjected to rubbing treatment in adirection of 45′ to the phase retardation axis (measured at 632.8 nm) ofthe polymer substrate (PK-1).

Formation of Optical Anisotropic Layer:

The alignment film was coated with a coating liquid containing 41.01 gof the discotic liquid crystal compound shown below, 4.06 g of ethyleneoxide-modified trimethylolpropane triacrylate (V#360, produced by OsakaYuki Kagaku K.K.), 0.35 g of cellulose acetate butyrate (CAB531-1,produced by Eastman Chemical Co.), 1.35 g of a photopolymerizationinitiator (Irgacure 907, produced by Ciba Geigy Co.), and 0.45 g of asensitizer (Kayacure DETX, produced by Nippon Kayaku Co.) dissolved in102 g of methyl ethyl ketone, by means of a #3 wire bar. The coated filmwas fixed to a metal frame, and was heated at 130° C. for 2 minutes in athermostatic oven to orient the discotic liquid crystal compound. Thetreated film was irradiated with UV for 1 minute at 130° C. by a highpressure mercury lamp of 120 W/cm to polymerize the discotic liquidcrystal compound, and was allowed to cool to room temperature. Thus, anoptical compensation sheet (KH-1) having an optical anisotropic layerwas prepared.

The optical anisotropic layer had a Re retardation value of 38 nm at 546nm. The angle (inclined angle) between the discotic plane and the firsttransparent supporting member face was 40° in average.

Discotic Liquid Crystal Compound

Preparation of Polarizing Plate:

The optical compensation sheet (KH-1) was bonded at the face of thepolymer substrate (PK-1) to the one face of the polarizer (HF-1) using apolyvinyl alcohol type adhesive. Separately, 80 μm thicktriacetylcellulose film (TD-80U, produced by Fuji Photo Film Co.) wastreated for saponification, and this film was bonded to the reverse faceof the polarizer.

The transmission axis of the polarizer and the phase retardation axis ofthe polymer substrate (PK-1) were placed parallel to each other, whereasthe transmission axis of the polarizer and the phase retardation axis ofthe commercial triacetylcellulose film were placed perpendicularly toeach other. Thus, a polarizer (HB-1) was prepared.

Example 2

Preparation of Polymer Substrate:

16 parts by mass of the retardation-increasing agent used in Example 1,80 parts by mass of methylene chloride, and 20 parts by mass of methanolwere placed in a mixing tank. The resulting mixture was stirred withheating to prepare a solution of the retardation-increasing agentsolution.

474 parts by mass of the cellulose acetate solution prepared in Example1 and 25 parts by mass of the retardation-increasing agent solution weremixed and stirred sufficiently to prepare a dope. The amount of theretardation-increasing agent added was 3.5 parts by mass per 100 partsby mass of cellulose acetate.

The thus-obtained dope was cast using a band-casting machine. When thefilm surface temperature had reached 40° C., the formed film was driedfor 1 minute. The formed film was stripped off, dried by a drying airstream at 140° C. Thus, a polymer substrate (PK-2) was prepared whichcontained the residual solvent at a content of 0.3 mass %.

The obtained polymer substrate (PK-2) had a thickness of 65 μm. Theretardation value (Re) at 590 nm of the polymer substrate was 8 nm, andthe retardation value (Rth) at 590 nm was 78 nm as measured by anellipsometer (M−150, manufactured by Nippon Bunko K.K.).

Preparation of Optical Compensation Sheet Having Optically AnisotropicLayer:

The polymer substrate (PK-2) was immersed in a 2.0N potassium hydroxidesolution (25° C.) for 2 minutes, neutralized with sulfuric acid, washedwith pure water, and dried. This PK-2 had a surface energy of 63 mN/m,as measured by a contact angle method.

Formation of Alignment Film:

The prepared PK-2 was coated with the alignment film-coating liquidhaving the composition shown below in an application amount of 28 mL/m²with a #16 wire bar coater. The coated film was dried by a warm airstream at 60° C. for 60 seconds and further at 90° C. for 150 seconds.

Composition of Aligning Film Coating Liquid

Parts by mass Modified polyvinyl alcohol of Example 1 10 Water 371Methanol 119 Glutaraldehyde (crosslinking agent) 0.5

The formed film was subjected to rubbing treatment in a directionparallel to the length direction of PK-2.

Formation of Optically Anisotropic Layer:

The alignment film was coated with a coating liquid containing 41.01 gof the discotic liquid crystal compound used in Example 1, 4.06 g ofethylene oxide-modified trimethylolpropane triacrylate (V#360, producedby Osaka Yuki Kagaku K.K.), 0.90 g of cellulose acetate butyrate(CAB551-0.2, produced by Eastman Chemical Co.), 0.23 g of celluloseacetate butyrate (CAB531-1, produced by Eastman Chemical Co.), 1.35 g ofa photopolymerization initiator (Irgacure 907, produced by Ciba GeigyCo.), and 0.45 g of a sensitizer (Kayacure DETX, produced by NipponKayaku Co.) dissolved in 102 g of methyl ethyl ketone, by means of #3.6wire bar. The coated film was heated in a thermostatic zone at 130° C.for 2 minutes to orient the discotic liquid crystal compound. The filmthus treated was irradiated with UV for 1 minute at 60° C. by a highpressure mercury lamp of 120 W/cm to polymerize the discotic liquidcrystal compound. The film was allowed to cool to room temperature.Thus, an optical compensation sheet (KH-2) having an opticallyanisotropic layer was prepared.

The optically anisotropic layer had a Re retardation value of 43 nmmeasured at 546 nm. The angle (inclination angle) between the discoticplane and the first transparent supporting member face was 42° inaverage.

Preparation of Polarizing Plate:

The optical compensation sheet (KH-2) was bonded to the one face of thepolarizer (HF-1) using a polyvinyl alcohol type adhesive. Separately, 80μm thick triacetylcellulose film (TD-80U, produced by Fuji Photo FilmCo.) was treated for saponification, and this film was bonded to thereverse face of the polarizer.

The transmission axis of the polarizer and the phase retardation axis ofthe polymer substrate (PK-2) were placed parallel to each other, whereasthe transmission axis of the polarizer and the phase retardation axis ofthe commercial triacetylcellulose film were placed perpendicularly toeach other. Thus, a polarizing plate (HB-2) was prepared.

Example 3

Preparation of Bend-Alignment Liquid Crystal Cell:

A polyimide film was provided as an alignment film on two glasssubstrates having an ITO electrode, respectively. The alignment filmswere subjected to rubbing treatment. Two glass substrates formed induplication were placed in opposition with the cell gap of 6 μm with therubbing treatment directions thereof parallel to each other. A liquidcrystal compound (ZLI 1132, produced by Merck Co.) having Δn of 0.1396was injected into the cell gap. Thereby, a bend-alignment liquid crystalcell was produced. The liquid cell had a size of 20 inches.

Two polarizing plates (HB-1) prepared in Example 1 were bonded to theboth faces of the above produced bend-alignment cell. In the bonding,the polarizing plates were placed with the optically anisotropic layersfacing respectively to the cell substrates with the rubbing-treateddirections of the liquid crystal cell and the elliptic polarizing platekept antiparallel.

A rectangular wave voltage of 55 Hz was applied to the liquid crystalcell in a normally-white mode of white display of 2V and black displayof 5V. Taking the transmittance ratio (white display/black display) asthe contrast ratio, the visual field angle was measured at 8 steps fromblack display (L1) to white display (L8) by a tester (EZ-Contrast 160D,manufactured by ELDIM Co.).

The results obtained are shown in Table 1 below.

TABLE 1 Visual field angle (Range of Liquid contrast ratio of 10 ormore, and Crystal no gradation reversal in black side) Display TopBottom Right and Left Example 3 80° 80° 80° (Note) Gradation reversal inblack side: Reversal between L1 and L2Evaluation of Light Leakage:

The backlight was turned on continuously for 5 hours under theenvironmental conditions of temperature 25° C. and relative humidity60%. The whole-area black display state was visually examined in a darkroom to evaluate the light leakage. As a result, no light leakage wasfound in the display screen of the liquid crystal display.

Example 4

A pair of polarizing plates were stripped off from a liquid crystaldisplay using. TN type liquid crystal cells (AQUOS LC20C1S, manufacturedby Sharp Corp.), and instead thereof, the polarizing plates (HB-2)prepared in Example 2 were bonded thereto, one on the observer's sideand another one on the backlight side with the optical compensationsheets (KH-2) facing the liquid cell with an adhesive. The transmissionaxis of the polarizing plate on the observer's side and the transmissionaxis of the polarizing plate on the backlight side are placed in anO-mode.

The prepared liquid crystal display was tested for the visual fieldangle in 8 steps from black display (L1) to white display (L8) with atester (EZ-Contrast 160D, manufactured by ELDIM Co.). The resultsobtained are shown in Table 2 below.

Comparative Example 1

A liquid crystal display using TN type liquid crystal cells (AQUOSLC20C1S, manufactured by Sharp Corp.) was tested for the visual fieldangle in 8 steps from black display (L1) to white display (L8) with atester (EZ-Contrast 160D, manufactured by ELDIM Co.).

The results obtained are shown in Table 2 below.

TABLE 2 Visual field angle (Range Liquid of contrast ratio of 10 ormore, Crystal and no gradation reversal reversal in black side) DisplayTop Bottom Right and Left Example 4 75° 43° 80° Comparative 70° 42° 80°Example 1 (Note) Gradation reversal in black side: Reversal between L1and L2Evaluation of Light Leakage:

The backlight was turned on continuously for 5 hours under theenvironmental conditions of temperature 25° C. and relative humidity60%. The whole-area black display state was visually examined in a darkroom to evaluate the light leakage. As the results, no light leakage wasfound in the display screen of the liquid crystal display of Example 4.However, framelike light leakage was observed in the display screen ofComparative Example 1.

Example 5

Preparation of Polymer Film:

16 parts by mass of the retardation-increasing agent used in Example 1,80 parts by mass of methylene chloride, and 20 parts by mass of methanolwere placed in a mixing tank. The mixture was stirred with heating toprepare a retardation-increasing agent solution.

464 parts by mass of the cellulose acetate solution prepared in Example1 and 36 parts by mass of the retardation-increasing agent solution weremixed and stirred sufficiently to prepare a dope. The amount of theretardation-increasing agent added was 5.0 parts by mass per 100 partsby mass of cellulose acetate.

The obtained dope was cast with a band-casting machine. When the filmsurface temperature had reached 40° C., the formed film was dried for 1minute. The formed film was stripped off, and dried by drying air streamat 140° C. The film was stretched by 30% in the width direction with atenter, and then dried with a drying air stream at 135° C. for about 20minutes. Thus, a polymer substrate (PK-3) was prepared which containedthe residual solvent at a content of 0.3 mass %.

The obtained polymer substrate (PK-3) had a thickness of 102 μm. Theretardation value (Re) at 590 nm of the substrate was 47 nm, and theretardation value (Rth) at 590 nm was 153 nm as measured by anellipsometer (M−150, manufactured by Nippon Bunko K.K.).

Preparation of Polarizing Plate:

The polymer substrate (PK-3) was bonded to one face of the polarizer(HF-1) using a polyvinyl alcohol type adhesive. Separately, 80 μm thicktriacetylcellulose film (TD-80U, produced by Fuji Photo Film Co.) wastreated for saponification, and this film was bonded to the reverse faceof the polarizer.

The transmission axis of the polarizer (HF-1) and the phase retardationaxis of the polymer substrate (PK-3) were placed parallel to each other,whereas the transmission axis of the polarizer and the phase retardationaxis of the commercial triacetylcellulose film were placedperpendicularly to each other. Thus, a polarizer (HB-3) was prepared.

Vertical Alignment Liquid Crystal Cell:

A pair of polarizing plates and a pair of retardation films werestripped off from a liquid crystal display using a vertical alignmentliquid crystal cells (VL-1530S, manufactured by Fujitsu Ltd.), andinstead thereof, the respective polarizing plates (HB-3) were bondedthereto with the polymer substrate (PK-3) facing the liquid cell sidewith interposition of an adhesive. The transmission axis of thepolarizing plate on the observer's side is directed vertically and thetransmission axis of the polarizing plate on the backlight side isplaced laterally in a cross-nicol arrangement.

The prepared liquid crystal display was tested for the visual fieldangles for 8 steps from black display (L1) to white display (L8) with atester (EZ-Contrast 160D, manufactured by ELDIM Co.). The resultsobtained are shown in Table 3 below.

Comparative Example 2

The liquid crystal display using a vertical alignment liquid crystalcells (VL-1530S, manufactured by Fujitsu Ltd.) was tested for the visualfield angles for 8 steps from black display (L1) to white display (L8)with a tester (EZ-Contrast 160D, manufactured by ELDIM Co.). The resultsobtained are shown in Table 3 below.

TABLE 3 Visual field angle (Range of contrast ratio of 10 or more, andno gradation reversal in black side) Liquid 45° from CrystalTransmission transmission Display axis direction axis direction Example4 >80° >80° Comparative >80°   44° Example 1 (Note) Gradation reversalin black side: Reversal between L1 and L2Evaluation of Light Leakage

The backlight was turned on continuously for 5 hours under theenvironmental conditions of temperature 25° C. and relative humidity60%. Thereafter the whole-area black display state was visually examinedin a dark room to evaluate the light leakage. As a result, no lightleakage was found in the display screen of the liquid crystal display ofExample 5. However, framelike light leakage was observed in the displayscreen of Comparative Example 2.

Example 6

PVA having an average degree of polymerization of 4000 and a degree ofsaponification of 99.8% was dissolved in water to obtain a 4.0% aqueoussolution thereof. This solution was cast through a tapered die onto acasting band, and dried to obtain a film having a width of 110 mm, and athickness of 120 μm at the left end and 135 μm at the right end beforestretching.

This film was stripped off from the casting band, and was stretchedobliquely at an angle of 45° in dry conditions. The film in this statewas immersed in an aqueous solution containing iodine (0.5 g/l) andpotassium iodide (50 g/l) at 30° C. for 1 minute, and then immersed inan aqueous solution containing boric acid (100 g/l) and potassium iodide(60 g/l) at 70° C. for 5 minutes. The film was washed with water in awater washing vessel at 20° C. for 10 seconds, and dried at 80° C. for 5minutes to obtain an iodine type polarizer (HF-4). The polarizer had awidth of 660 mm and a thickness of 20 μm at both of the right and leftedges.

A polarizing plate (HB-4) was prepared in the same manner as in thepreparation of the polarizing plate in Example 2 except that thepolarizer (HF-4) was used in place of the polarizer (HF-1).

Example 7

PVA having an average degree of polymerization of 2500 and a degree ofsaponification of 99.5% was dissolved in water to obtain a 50% aqueoussolution. This solution was cast through a die with a taper on a castingband, and dried to obtain a film having a width of 300 mm, and athickness of 100 μm at the left end and 115 μm at the right end beforestretching.

This film was stripped off from the casting band, and the film in thisstate was immersed in an aqueous solution containing iodine (0.2 g/l)and potassium iodide (60 g/l) at 30° C. for 5 minutes. The film wasimmersed in an aqueous solution containing boric acid (100 g/l) andpotassium iodide (30 g/l) with stretching in an oblique direction at anangle of 45° at 60° C. for 10 minutes. The film had a width of 1,900 mmand a thickness of 15 μm at both of the right and left edges.

The film was immersed in a water washing vessel at 20° C. for 10seconds, and further immersed in an aqueous solution containing iodine(0.1 g/l) and potassium iodide (30 g/l) at 30° C. for 15 seconds. Thefilm was dried at room temperature for 24 hours to obtain an iodine typepolarizer (HF-5).

A polarizing plate (HB-5) was prepared in the same manner as in thepolarizing plate in Example 2 except that the polarizer (HF-5) was usedin place of the polarizer (HF-1).

Evaluation of Production Yield of Polarizing Plate:

The number of sheets having a size of 219.0×291.4 mm which could bepunched out from the polarizing sheet was measured. The size of thepolarizing plate was 650 mm in width and 1000 mm in length incorrespondence with Comparative Example 1.

From the polarizing plates of Examples 6 and 7, nine sheets could bepunched out for 14.1-inch LCD. This yield is much higher than the yieldof six sheets of commercial polarizing plates.

Examples 8 and 9

The polarizing plates of Examples 6 and 7 were evaluated for the visualfield angle and the marginal irregularity in the same manner as inExample 4 except that the polarizing plate of example 6 or 7 (HB-4 orHB-5) was used in place of the polarizing plate (HB-2). Both of thepolarizing plates were excellent.

Example 10

A polarizing plate (HB-6) was prepared in the same manner as in Example2 except that the polymer substrate (PK-2) was replaced by a polymersubstrate (PK-4) having a thickness of 80 μm and adjusted to have thesame retardation value.

The polarizing plate was evaluated for the visual field angle and themarginal irregularity in the same manner as in Example 4 except that thepolarizing plate (HB-6) was used in place of the polarizing plate(HB-2). The polarizing plate (HB-6) caused much less marginalirregularity than that of Comparative Example 1 and was excellent, butcaused slight marginal irregularity in comparison with the polarizingplate (HB-2). Thereby, it was confirmed that the polymer substrate ispreferably thinner. The visual field angle was excellent.

INDUSTRIAL APPLICABILITY

The present invention enables optical compensation of an optical cellwithout adverse effect, and suppression of marginal increase oftransmission, by the use of a polarizing plate comprising a polarizerhaving a thickness of 10–25 μm and an optically anisotropic layercomprising a liquid crystal compound.

Further, the production yield of the polarizing plate could beremarkably improved by providing a large polarizing plate in which atransmission axis is present neither in the length nor in the breadthdirection.

1. A polarizing plate comprising a polymer film, a polarizer, a polymersubstrate, and an optically anisotropic layer comprising a liquidcrystal compound, laminated in this order, wherein the polarizer has athickness of 10 to 25 μm.
 2. The polarizing plate as claimed in claim 1,wherein the polymer substrate has a thickness of 30 to 70 μm.
 3. Thepolarizing plate as claimed in claim 1, wherein the polarizer has athickness of 10 to 20 μm.
 4. The polarizing plate as claimed in claim 1,wherein the polymer comprises cellulose acetate.
 5. The polarizing plateas claimed in claim 1, wherein the polymer substrate comprises celluloseacetate.
 6. The polarizing plate as claimed in claim 1, wherein theliquid crystal compound used in the optical anisotropic layer is adiscotic liquid crystal compound, the plane of the discotic structuralunits is inclined relative to the surface of the polymer substrate, andthe angle between the plane of the discotic structural units and thesurface of the polymer substrate changes in the direction of the depthof the optically anisotropic layer.
 7. A liquid crystal displaycomprising a liquid crystal cell, and two polarizing plates placed onboth faces of the liquid crystal cell, wherein at least one of thepolarizing plates is the polarizing plate as claimed in claim
 1. 8. Theliquid crystal display as claimed in claim 7, wherein the liquid crystalcell is of an OCB mode, a VA mode, or a TN mode.
 9. A polarizing platecomprising a protective film, a polarizer, and a poly film substrate,laminated in this order, wherein the polarizer has a thickness of 10 to25 μm, and the polymer film has a Re retardation value defined by thefollowing formula (I) in a range of 20 to 70 nm, a Rth retardation valuedefined by the following formula (II) in a range of 70 to 400 nm:Re=(nx−ny)×d  (I)Rth=[(nx+ny)/2−nz]×d  (II) wherein nx and ny are refractive indexes of aslow axis and a fast axis in plane of the polymer film substrate, and nzis a refractive index of a thickness direction of the polymer filmsubstrate.